Occasionally we like to remind you that you should not mess with nature because it is weirder and scarier than you. Case in point: duck dick.

Researchers who are probably a huge amount of fun at parties have been studying duck penises for a while, trying to figure out a) why they are so fucking disturbing (please be warned that watching the video above may ruin your enjoyment of ducks permanently); and b) why they even exist. Most birds don’t have penises — both sexes have cloaca, or genital openings. Others, like chickens, sort of have penises but they’re clearly vestigial and not good for much. But ducks, a few other waterfowl, and large birds like ostriches are packing. Why?

As Ed Yong explains at National Geographic, the way it happens is even weirder than you might have been expecting: The penis cells in most birds actually kill themselves off during development.

In ducks, the genital tubercle keeps on growing into a long coiled penis, but in the chicken, it stops around day 9, while it’s still small. Why? [University of Florida scientist Martin] Cohn expected to find that chickens are missing some critical molecule. Instead, his team found that all the right penis-growing genes are switched on in the chicken’s tubercle, and its cells are certainly capable of growing.

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It never develops a full-blown penis because, at a certain point, its cells start committing mass suicide. This type of ‘programmed cell death’ occurs throughout the living world and helps to carve away unwanted body parts—for example, our hands have fingers because the cells between them die when we’re embryos. For the chicken, it means no penis. “It was surprising to learn that outgrowth fails not due to absence of a critical growth factor, but due to presence of a cell death factor,” says Cohn.

Of course, that just explains the mechanism, not the reason. That’s a little harder to determine for sure. Could be a side effect of other changes that are adaptive enough to make up for the inconvenience of having to mash your cloaca together in order to reproduce. Could be that wangless birds are more aerodynamic. Could be that females of most species got sick of how rapey their dick-swinging males were getting. (Did I ruin ducks for you yet? Because wow, they really make this option seem plausible.)

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Anyway, it’s really worth checking out the rest of Yong’s piece, which features more penis facts and a chart that looks like one of Lady Gaga’s more aggressive jewelry racks.